But its worth reading that article and seeing the difference between Dual Shift, Dual Standby and Dual Active. Especially as a few dual sim phones being sold by Nokia and Sony are actually Dual shift and tbh i would never buy a dual shift phone, may as well buy two separate phones. that'd work better.

You don't have to turn it off but you have to switch between sim manually. So in effect you are only using one sim at any one time. You then have to turn off that sim so it doesn't work at all to use the other one.

As long as you buy a dual standby/dual active phone you're fine as both sims work at the same time. And tbh most phones are dual standby/active